Set default mrac value with swerv.config

[olofk]

In most cases, SweRV is used in an environment where the SoC designer already knows which parts of the memory space that is used for RAM and devices. For that reason it would be good if we could set the MRAC default value at compile-time so that we don't have to do a csr write+flush in every bootloader and program/OS startup code.

[agrobman]

I'm afraid this request contradicts the CPU spec and our internal customers requirements.